Tapered core fiber manufacturing methods

ABSTRACT

Tapered core fibers are produced using tapered core rods that can be etched or ground so that a fiber cladding has a constant diameter. The tapered core can be an actively doped core, or a passive core. One or more sleeving tubes can be collapsed onto a tapered core rod and exterior portions of the collapsed sleeving tubes can be ground to provide a constant cladding diameter in a fiber drawn from the preform.

FIELD

The disclosure pertains to optical fibers with tapered cores.

BACKGROUND

Optical fibers are generally fabricated so as to have substantiallyconstant core and cladding diameters. These constant diameters result inlow propagation losses, and permit relatively straightforwardconnecterization of fiber cables and fiber splicing. However, taperedfibers can be useful as well. For example, a tapered fiber can be usedto suppress higher order modes, or to expand a mode field diameter toimprove mode matching between different fibers. Tapering a fiber istypically based on a drawing process so that a fiber core and claddingare tapered in the same manner, and a ratio of core and claddingdiameters is unchanged by tapering. Some applications of fibertechnology can be better addressed with independent control of fibercore and cladding diameters.

SUMMARY

Disclosed herein are representative optical fibers, fiber preforms, andassociated methods that can provide fibers with tapered cores andconstant diameter claddings. In some examples, an optical fibercomprises a tapered core that extends along a propagation axis. An innercladding surrounds the core and extends along the propagation axis,wherein the tapered core and the inner cladding have respectivecross-sectional areas A_(CORE), A_(INNER), and a ratioA_(CORE)/A_(INNER) varies along the propagation axis. In typicalexamples, the inner cladding has a constant cross-sectional area and thetapered core defines a few mode optical fiber. In some examples, thecore is doped with a rare earth element. In other embodiments, an outercladding surrounds the inner cladding, wherein the inner cladding andthe core are configured to guide optical radiation. In some examples,the tapered core and the inner cladding have circular cross-sectionalareas. In representative examples, at least a portion of the taperedcore defines a single mode waveguide and a radius of the core varieslinearly along the propagation axis. In other examples, a core radiusvaries periodically or quasi-periodically along the propagation axis. Inconvenient examples, the core is centered with respect to the innercladding.

Fiber preforms typically comprise concentric cylinders or layers ofmaterials suitable for forming optical waveguides. A central cylindercan serve to define a waveguide core, and layers exterior to the centralcylinder can serve to define waveguide cladding layers with generallylower refractive indices than that of the central core-forming portion.In many preform manufacturing methods, one or multiple sleevingprocesses take place, in which one or more glass tubes are collapsedonto a glass rod, glass tube, or a stack of glass rods and/or tubes. Asused herein, a core rod is defined as a core waveguide material shapedso as to be suitable for forming into a waveguide core, and may includeone or more surrounding cladding materials.

Tapered fiber preforms comprise a tapered core rod, and at least onesleeving tube is situated about the core rod and collapsed toward thetapered core rod. In some examples, a plurality of silica grains issituated between the tapered core rod and an interior surface of the atleast one sleeving tube. In still other examples, a plurality ofsleeving tubes is situated about the core rod and collapsed toward thetapered core rod. In some embodiments, the at least one sleeving tubeincludes a plurality of indentations or protrusions at an exterior orinterior surface. In some examples, the tapers of the tapered core rodinclude a plurality of neck regions, and the protrusions of the sleevingtube are situated at respective neck regions. In further embodiments,the sleeving tube includes a plurality of apertures in a wall of thesleeving tube. In other examples, the sleeving tube comprises a firstsection and a second section situated to provide a gap between the firstsection and the second section.

Methods comprise providing a core rod and collapsing an inner sleevingtube onto the core rod. The sleeved core rod is then tapered bymachining, etching, or drawing. In some representative examples, atleast one outer sleeving tube is collapsed onto the sleeved, taperedcore rod. The outer sleeved tapered core rod is processed to provide aconstant cross-sectional area or a constant diameter. Typically,processing includes at least one of etching or machining the outersleeving tube.

In other examples, methods include situating a tapered core rod in acladding tube and depositing silica grains in the cladding tube. Thesilica grains and the cladding tube are fused to the tapered core rod toform a fused fiber preform. In some examples, the fused fiber preform isdrawn so as to produce an optical fiber. In typical examples, the fusedfiber preform is drawn so as to produce a fiber with a constant claddingdiameter. In other representative embodiments, a vacuum is applied tothe silica grains prior to or during fusing. The tapered core rod can bean actively doped core rod or an undoped core rod.

Additional methods comprise situating a glass structure including thecore within a cladding tube and applying a modulated feed ratedifference between the structure containing the core and the claddingtube. The core structure and the cladding tube are drawn to form anoptical fiber, wherein at least one of a fiber core diameter and a fibercladding diameter is at least partially determined by the feed ratedifference. In particular examples, the feed rate difference is selectedso that the structure including the core and cladding is drawn so thatthe optical fiber has a tapered core. Typically, a time-varying feedrate is applied. In some embodiments, at least one capillary tube issituated within the cladding tube, and the core structure, the capillarytube, and the cladding tube are drawn to produce the optical fiber. Infurther examples, the cladding tube interior is at least partiallyfilled with silica grains, and the silica grains are fused as the coretube, the capillary tube, and the cladding tube are drawn to form theoptical fiber.

Other methods comprise situating a core rod having a tapered core in acladding tube, and drawing the core rod and the cladding tube so as toform an optical fiber. In some examples, the tapered core is doped withan active dopant. In further embodiments, the core rod and the claddingtube are drawn so that the optical fiber has a constant claddingdiameter. In still other examples, the core rod has a constant outsidediameter.

Methods of producing a fiber comprise situating a core tube within asleeving tube and drawing the core tube and the sleeving tube such thatthe core tube and the sleeving tube have a feed rate difference. In someexamples, the feed rate difference is selected to produce a tapered corefiber, a tapered core fiber and a constant cladding diameter, or atapered cladding exterior.

The foregoing and other features and advantages of the disclosedtechnology will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a fiber preform that includes a coretube, a cladding tube, and a plurality of capillary tubes situatedbetween the core tube and the cladding tube.

FIG. 2 illustrates processing of a preform similar to that shown in FIG.1.

FIG. 3 is schematic block diagram of a representative method of making atapered core optical fiber.

FIGS. 4A-4B illustrate fiber preforms for fabricating tapered coreoptical fibers.

FIG. 5 is a cross-sectional view of a core rod preform.

FIGS. 6A-6B illustrate fiber preforms configured to produce a varyingcore diameter and a constant cladding diameter.

FIG. 7A is a sectional view of a representative arrangement for a fiberpreform that includes a tapered core.

FIG. 7B illustrates a sleeving tube having notches and protrusions on anexterior wall.

FIG. 8 illustrates a tapered core rod illustrating reduction of anexterior taper by grinding.

FIG. 9 illustrates a tapered core rod having a resist layer applied forreduction of an exterior taper by etching.

FIG. 10 is a schematic diagram illustrating formation of a tapered fiberby varying a feed rate of a core tube or rod and a feed rate of acladding tube.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatus are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved. Any theories of operation are to facilitateexplanation, but the disclosed systems, methods, and apparatus are notlimited to such theories of operation.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

Optical fibers or other optical waveguides are generally based on avariation of refractive index as a function of distance from apropagation axis. Such refractive index variations include so-calledindex steps such as those associated with typical step index fibers andcontinuous variations such as those associated with typical gradientindex fibers. Many convenient examples are based on optical fibershaving circular cross-sections. Such fibers generally include a centralcore that is surrounded by a cladding region and the core and claddingare selected to provide guided wave propagation. In the examplesdisclosed below, optical fibers, optical fiber sections, preforms, andwaveguide devices are shown as extending along linear axes. It will beappreciated that fibers and preforms can be arranged on curved,segmented, or axes of other configurations. Typically, such devicesextend along propagation axes along which optical radiation propagatesin the device, and such axes can be linear or curved.

In some examples, multimode or single mode devices are described, but bysuitable selection of device characteristics such as core and claddingrefractive indices (or refractive index difference) and dimensions,multimode or single mode devices can be fabricated. To obtain singlemode propagation, fiber characteristics are selected so that theso-called V-number V=πNAd_(CORE)/λ is less than about 2.4, wherein λ isa free space wavelength of radiation to be propagated in the device,d_(CORE) is a core diameter, and NA is a device numerical aperture.Device mode field diameters (MFD) can also be selected based on devicecharacteristics, as MFD=d_(CORE)(0.65+1.619/V^(3/2)+2.879/V⁶). Theserelationships are suitable for fiber devices having circularcross-sections, but similar considerations are applicable for othercross-sectional shapes. While considerable variation in fiber dimensionsis possible, in typical examples, single mode fibers for use atwavelengths between about 500 nm and 1500 nm have core diameters ofbetween about 3 μm and 20 μm, while multimode fibers have core diametersof between about 10 μm and 500 μm. Propagation characteristics can bebased on step index or gradient index designs. For convenientillustration, sectional views of fibers and preforms are provided. Whilein many useful examples, fiber and preform cross-sections are circular,oval, elliptical, polygonal or other cross-sections can be used. Inaddition, in some examples, stress rods or other core features can beprovided.

The disclosed examples generally pertain to fibers that have a singlecore surrounded by a cladding layer. However, in other examplesso-called double clad fibers can be formed. Double clad fibers generallyinclude a core surrounded by an inner cladding which is in turnsurrounded by an outer cladding. Refractive indices and refractive indexprofiles for these layers can be selected to provide selected waveguidecharacteristics. In some examples, double clad fibers include anactively doped core that can be configured to support single modepropagation. The active core and the inner cladding can serve to guidepump radiation into the active gain element of the core. Typically thecore has a higher refractive index that the inner cladding, and theinner cladding has higher refractive index than the outer cladding. Indouble clad fibers with actively doped cores, the core and innercladding can be decentered with respect to each other so as to moreefficiently couple pump radiation from the inner cladding into the core,but other configurations of inner clad and core can be used. Other fibertypes and associated preforms can be made in the same manner, includingpolarization retaining fibers that generally include stress elementssituated in a cladding layer so as to produce birefringence.

Representative fiber preforms, core rods, tapered core rods, and othercomponents for fiber preforms and fibers, and optical fibers based onsuch preforms are described below. Preforms can be made by modifiedchemical vapor deposition (MCVD) or other processes. Typically, amixture of oxygen, silicon tetrachloride (SiCl4) and materials such asgermanium tetrachloride (GeCl4) or rare earth dopants are introducedinto a silica glass tube, which is rotated while heated to about1500-1600 C with a torch. An inner surface of the glass tube is coated,and a layer with higher refractive index is formed which can be drawninto a fiber core. Typically, the glass tube is collapsed by furtherheating to form a core rod. Other deposition methods such outside vapordeposition (OVD), direct nanoparticle deposition, or others can also beused as well.

Representative methods for producing tapered core optical fibers orwaveguides can be based on manipulation of fiber properties during fiberdrawing using varied draw speeds applied to a drawn fiber or feed ratesapplied to one or more preform structures such as sleeving tubes,cladding tubes, core tubes, or capillary tubes. As used herein, a feedrate is associated with the speed of a fiber preform supplied to or in adrawing furnace, and draw speed is a speed of the drawn fiber. A ratioof feed rate and draw speed can be used to define fiber thickness

In typical examples of such methods, a fiber preform or other assemblyfrom which a fiber is to be drawn is initially untapered, and taper isdeveloped during drawing. In other representative methods, a preform isbased on a tapered core rod that can be fabricated by a machining,grinding, etching, or other process. In some examples, a tapered corerod is processed so as to have a substantially constant cross-sectionalshape with an internally varying core. Preforms with tapered cores aretypically drawn so that taper lengths in the tapered core are extendedby factors of 100-2000. For example, a core rod having a 5 mm length canbe drawn into a fiber having a length of about 5 m. As disclosed herein,multiple tapers can be provided in a single preform, and separated afterperform is drawn into a fiber.

In some examples, a tapered core is situated within a sleeving tube thatis filled or packed with silica grains, or grains of other glassymaterials. The grains can be packed, dried, and degassed prior to fiberdrawing in which the sleeving tube and the grains are fused onto thecore rod. The locations of the beginning and end of a taper in a fibermay not be apparent, but tapers doped with a fluorescent gain speciescan be viewed directly based on fluorescence emitted in response to pumpradiation applied to the core. For example, fluorescence can be observedin a direction perpendicular to a fiber axis. In some cases, a pluralityof sleeving tubes can be used.

Fibers can include tapered cores with claddings that have a constantcladding diameter. In general, core tapers can be provided without acorresponding taper in a fiber exterior surface such as a claddingsurface, and dimensions and shapes of a fiber exterior can be selectedindependently of core taper. In the examples provided below, taperedcores in fibers having generally circular cross-sectional areas aredescribed for convenient illustration and because they tend to besuitable for a wide variety of applications. However, similar taperedcores can be provided in fibers of other cross-sectional areas.

Core rods can be formed using direct nanoparticle deposition, and placedwithin a sleeve that is collapsed onto the core rod. A sleeved core rodis more robust that an unsleeved core rod, and the sleeved rod can betapered by drawing, grinding, etching, or other processes for inclusionin a fiber preform. MCVD or other processes can also be used.

In some examples, optical fibers comprise a core that extends along apropagation axis and a cladding that surrounds the core and extendsalong the propagation axis. The core and cladding have respectivecross-sectional areas A_(CORE), A_(CLAD), and a ratio A_(CORE)/A_(CLAD)of these cross-sectional areas is a function of position z along thepropagation axis so that A_(CORE)/A_(CLAD)=f(z), wherein f(z) is not aconstant. In other examples, fibers include a core, an inner cladding,and an outer cladding that are all configured to propagate opticalradiation. The ratio functions or the variation in the cross-sectionalareas or core or clad effective diameters can correspond to linear ornonlinear functions, trigonometric functions, periodic functions,polynomial functions, step functions, or other functions. Inrepresentative embodiments, the cladding has a constant cross-sectionalarea or diameter. In other embodiments, the core is doped with a rareearth element. In further embodiments, the core is tapered along theaxis and the core and the cladding have circular cross-sectional areas.

In some examples, core or cladding cross-sectional areas or radii varysinusoidally or otherwise at a fixed spatial frequency, and in otherexamples, periodic variation scans include a plurality of variationswith different periods, such as a chirped spatial frequency thatincreases or decreases along the propagation axis. Such variations arereferred to herein as quasi-periodic.

Preforms for forming optical fibers comprise a core having a corecross-sectional area and extending along a preform axis. A cladding hasa cladding cross-sectional area and extends along the preform axis,wherein a ratio of the core and cladding cross-sectional areas variesalong the preform axis. In some examples, at least one of the preformcore and the preform cladding is doped so that a preform core refractiveindex is greater than a preform cladding refractive index. In otherexamples, the preform core and the preform cladding have substantiallycircular cross-sectional areas.

Example 1

A sectional view of a fiber preform 100 is illustrated in FIG. 1. Thepreform 100 includes a core tube 102 having an interior void 104. Thecore tube 102 is typically doped with at least one active laser speciessuch as Nd or other rare earth element, or other suitable laser dopant.The core tube 102 is surrounded by a cladding tube 106, and one or morecapillary tubes such as representative capillary tube 108 are situatedin a space 110 between the cladding tube 106 and the core tube 102. Thecore tube 102 and the cladding tube 106 generally are situated so as toextend along a central axis for ease in drawing the fiber preform 100into a fiber. The diameter and wall thickness of the cladding tube 106and dimensions and dopings of the core tube 102 are generally selectedbased on intended fiber characteristics such as core diameter, claddingdiameter, and core doping density and doping density profile in thecore. The capillary tubes such as the capillary tube 108 may alsocontribute to cladding thickness, but these capillary tubes can beselected to have sufficiently low wall thicknesses so as to make only alimited contribution to the fiber cladding.

Example 2

FIG. 2 illustrates a fiber preform 200 such as that shown in FIG. 1having an unprocessed portion 202, a tapered portion 204, and an opticalfiber portion 206. The preform includes a core tube 210 having a centralvoid 212, and a cladding tube 214. One or more capillary tubes such asrepresentative capillary tube 216 are situated in cavity 218 defined bythe core tube 210 and the cladding tube 214. In the tapered portion 204,the core tube 210, the capillary tube 216, and the cladding tube 214taper to as to have smaller diameters, with the core tube 214 and thecapillary tube 216 collapsing on the core tube 210. The optical fiberportion 206 includes a fiber core 222 defined by the core tube 210 and afiber cladding 224 defined by the cladding tube 224, with somecontribution from the collapsed capillary tube 216. Characteristics ofthe fiber portion 206 can be determined by selection of core tube andcladding tube materials, dimensions, and dopings as well as pressuresapplied to the interiors of the core tube 210, the cladding tube 214,and the capillary tube 216. For example, the fiber core 222 can betapered.

Example 3

Fiber preforms containing either core rods with voids, core tubes, orcapillary tubes situated between a core rod and a cladding tube as shownin FIGS. 1-2 can be processed to form variably tapered fibers based onpressures coupled to the interiors of capillary tubes and/or pressurescoupled to a void in a core rod, in addition to feed rate differencesbetween the parts. In some examples, application of a pressure to a corerod or a cladding tube can be used to provide a selected feed rate.Fiber draw parameters such as draw speed and temperature can also beselected independently or in conjunction with applied pressures and feedrates. FIG. 3 illustrates a representative method 300 that includesobtaining a fiber preform at 302, wherein the fiber preform typicallyincludes a cladding tube that contains a core rod or tube, and one ormore capillary tubes. A first pressure and feed rate to be provided to avolume defined by a cladding rod interior surface is selected at 304. Asecond pressure and feed rate is selected for application to one or morevolumes corresponding to capillary tube interiors at 306, wherein thecapillary tubes are situated within the volume defined by the claddingrod interior surface. A third pressure can be selected for applicationto a volume corresponding to a void in the core tube at 308. Thesepressures and feed rates are generally selected to be time-varyingduring a fiber drawing process so that one or both of a core or claddingdiameter can be varied, and adjustments to fiber drawing can be madeduring processing. Typically, core tube feed rate can be controlled orselected based on core tube pressure, and clad tube feed rate can bebased on clad tube pressure. At 310, the selected preform is pressurizedaccording to the selected pressures, and fiber drawing is performed at312. At 314, the drawn fiber is assessed based on core diameter,cladding diameter, taper extent, taper rate, or other characteristics sothat one or more of the pressures and/or other draw parameters can bechanged appropriately. At 316, if adjustment of fiber draw parameters isneeded, parameters such as pressures and feed rates can be re-selectedat 304, 306, 308, or other process parameters such as draw rate, drawtemperature, and tension can be adjusted.

While first, second, and third pressures can be selected, in someexamples, pressure differences are selected. For example, a coretube/cladding tube pressure difference can be selected, and a capillarytube pressure can be selected based on the core tube or cladding tubepressures. Pressures or pressure differences can be based on applicationof an inert gas such as helium, or other suitable gas, or one or morevolumes can be evacuated or partially evacuated. By varying pressuresand pressure differences together with the feed rates, core or claddingdiameters can be adjusted. For example, increasing a pressure applied tothe interior of the core tube tends to increase core diameter so that byincreasing and decreasing core tube pressures, a variably tapered corecan be produced. Cladding diameter can also be varied by control ofapplied pressures, or cladding diameter can be held substantiallyconstant while a core is tapered by suitable pressure adjustments. Inother examples, feed rates can be changed during drawing to vary corearea, and cladding diameter can be constant or varied as well.

Example 4

A portion 400 of an alternative preform for forming fibers with taperedcores is illustrated in FIG. 4A prior to a sintering process. Thepreform 400 includes a core rod 402 having a doped central region 406and an undoped outer portion 404. In some examples, one or both of theregions are undoped, have different dopings, or have the same dopings.Dopants can be provided to provide suitable refractive index differencesto establish preferred optical mode characteristics, or gain speciessuch as rare earth elements can be introduced to provide optical signalamplification. The tapering of the core rod 402 can be provided byheating in a O₂-H₂ flame or other heat source and applying a varyinglongitudinal tension. The core rod 402 is situated within a claddingtube 410, and a volume 413 between an interior wall 412 of the claddingtube 410 and the core rod 402 is partially or completely filled with aSiO₂ grains or other glass or glassy material. The preform 400 can beprocessed so that the SiO₂ grains are sintered and a glass tube 416 isformed around the core rod 402 as shown in FIG. 4B.

The grains or other glassy material can be sintered in a fiber drawtower, and processed from bottom up to collapse the cladding tube sothat the grains are retained between the core rod and the cladding tube.In this way, the grains are held in place during fiber drawing. Smallgrains tend to produce fewer bubbles but may be difficult to use ifvacuum pumping around grains is desired as some grains may cause pumpdamage. In the above examples, silica grains or other glassy materialsare used as fillers, but in other examples, sol-gel materials can beused.

Example 5

In an alternative example illustrated in FIG. 5, a core rod preform 502is formed by situating one or more silica tubes 504-510 in a largersilica tube 511. A space 512 exterior to the silica tubes 504-510 isfilled or partially filled with silica granules or other glassymaterial. The silica tubes 504-510 can be filled with silica granulesand a predetermined concentration of a rare earth oxide and aluminumoxide. Concentrations can be selected to provide suitable refractiveindices, optical gain, or for other purposes. A core rod is formed fromthe core rod preform 502 by drawing at a temperature of 1500-2200° C.with a variable longitudinal tension or draw rate so that the core rodhas a variable taper. In some examples, only a single silica tube isprovided and is at least partially filled with a rare earth/aluminumoxide mixture. The resulting core rod can be situated at a center of asecond silica tube 507 and undoped silica used to fill or partially filla volume defined by the second silica tube and the core rod to form asecondary preform. The secondary preform is then preheated andevacuated, and then drawn into a fiber. In some examples, the core rodis situated in a silica tube having a diameter of between 17 mm and 21mm, and the fiber has a diameter between about 50 μm and 2 mm, and fibercore diameter is between about 5 μm and 20 μm. In the above examples,silica grains or other glassy materials are used as fillers, but inother examples, sol-gel materials can be used.

Example 6

With reference to FIG. 6A, a tapered core rod 600 includes a centraltapered region 602 contained within a perimeter cladding layer 604. Thetapered core rod 600 can be formed by heating an untapered core rod withan O₂-H₂ flame or other heat source and drawing the core rod. Thecentral tapered region 602 can include one or more dopants to provide anincreased refractive index with respect to the perimeter cladding layer604 to define guided optical modes, and/or to produce optical gain inresponse to pumping by a pump source such as one or more laser diodes.One or more sleeving tubes such as representative sleeving tubes 606,608 can be situated about the core rod 600 and collapsed onto the corerod 600 to become a fiber preform by heating.

FIG. 6B illustrates a preform 610 obtained by collapse of the sleevingtubes 606, 608 onto the core rod 600 as shown in FIG. 6A. The core rod600 is situated within collapsed sleeving 614. Due to the varyingdiameter of the core rod 600, an exterior surface 612 of the preform 610has a corresponding tapered or otherwise varying shape. Variation in thediameter of the preform 610 and the shape of the surface 612 typicallydepends on the taper of the core rod 600 and the number and dimensionsof the sleeving tubes. The exterior surface 612 can be shaped to providea round, polygonal, or other cross-section. For example, the preform 610can be ground to a so as to have a circular cross-section defined by agrinding surface 616. The resulting preform can be processed into anoptical fiber with a conventional drawing process to produce fibershaving tapered cores and constant cladding diameters.

The tapered core rod 600 can have slightly elliptical or othernon-circular cross-sections, and collapse of sleeving tubes can beassociated with bubbles between the collapsed sleeving tubes 606, 608and the core rod 600. Bubbles tend to be less likely with thin walledsleeving tubes and sleeving tubes with low melting temperatures. Inaddition, bubble formation can be reduced using sleeving tubes selectedso that gaps between sleeving tubes and the core rod 600 are controlledto limit gap size.

Example 7

With reference to the sectional view of FIG. 7A, a fiber preform can beformed with a tapered core rod 702 and a sleeving tube 708 that definean interior volume 706. The tapered core rod 702 can include a centraldoped region 700 that can be provided with an active dopant to produceoptical gain, or a dopant selected to provide an intended refractiveindex. The sleeving tube 708 can include one or more apertures such asaperture 710 that can permit evacuation of the interior volume 706. Theaperture 710 can be circular, elliptical, polygonal or other shape, anda plurality of such apertures can be provided along a length of thesleeving tube 708. In other examples, the aperture 710 can be providedas a periodic or aperiodic series of slits extending parallel to apreform axis. In another alternative, the sleeving tube 708 can have twoparts that are situated so as to define the aperture 710 as a gapbetween the parts, or a single sleeving tube can have a longitudinallyextending slit. Such slits and apertures can permit gases trappedbetween the sleeving tube 708 and the tapered core 702 to exit as thesleeving tube 708 is collapsed, thereby reducing the likelihood ofbubble formation. Bubbles can also be reduced by filling any volumes ofconcern with helium gas. In another example, longitudinally extendingcavities such as cavities 712, 714 can be provided, and permit trappedgases to be removed as well. In some examples, such cavities are coupledto a vacuum system. In other examples, slits, apertures, or cavities maybe situated at one or more locations about the sleeving tube asconvenient, and can be symmetrically or asymmetrically placed.

Example 8

With reference to FIG. 7B, a representative multi-part preform includesa tapered core rod 752 having a tapered core 753 (typically rare earthdoped), and an inner sleeving tube 754 that are situated along an axis755. A second sleeving tube 756 is situated about the tapered core rod752 along the axis 755 to be collapsed onto the tapered core rod 752.The second sleeving tube 756 includes alternating protrusions 760 andnotches 760 that can be aligned with respect to the tapered rod 752 sothat the protrusions are situated at narrower portions or “necks” 770 ofthe tapered core rod 752 and the notches 761 are situated at relativelywider portions 771 of the tapered core rod 752. Dimensions of theprotrusions and notches can be selected to at least partially compensatetaper so that after the sleeving tube 754 is collapsed onto the taperedcore rod 752, diameter variations tend to be reduced. In other examples,the sleeving tube 754 can be situated so as to increase diametervariations.

Example 9

In another example, a tapered core rod can be ground or otherwiseprocessed to have a substantially constant, untapered exterior surface.Referring to FIG. 8, a tapered core rod 800 includes a tapered core 802and an overcladding 804 having an exterior surface 806 whose shape isindicative of the taper of the tapered core 802. A trim line 808indicates portions of the tapered core rod 800 that can be removed bygrinding or other process to produce a preform with an untaperedexterior, typically a cylindrical exterior. However, the exteriorsurface can be ground, polished, or otherwise processed to produce arectangular, elliptical, polygonal, or other cross-section. Such apreform can then be drawn to produce a tapered core fiber with asubstantially constant outside diameter. Typically, such a ground corerod is provided with one or more sleeves that are collapsed to produce apreform. The untapered exterior surface of the core rod tends to reducebubble formation as the sleeving tubes are collapsed.

Example 10

In other examples, a tapered core rod can be processed to remove orreduce an exterior taper using an etching process. Referring to FIG. 9,a tapered core rod 900 includes a tapered core 902 and a taperedovercladding 904. To reduce taper of an exterior surface 907 of thetapered core rod 900, resist such as a photoresist is applied at smallerdiameter portions of the tapered core rod 900, and the tapered rod 900is exposed to an etchant such as an HF solution. Photoresist can beapplied and patterned by coating the exterior surface 907 and thenrotating the coated tapered rod with a suitable pattern situated betweenthe tapered coated rod and an exposing light source. In other examples,the surface 907 can be covered with a protective film such as apolytetrafluoroethylene tape, or tape of other relatively unreactivematerial. After etching, the core rod can be untapered or only slightlytapered, and can be sleeved to build up a suitable preform.

Example 11

Preform, core, cladding, and other dimensions can be selected based oncharacteristics of a fiber to be produced. In typical examples, fibershaving core/cladding diameters ranging from 2 μm/400 μm to 60 μm/1000 μmare formed. In some examples, a core rod has a length of 10-100 mm, anda diameter of between 5 mm and 25 mm. An untapered core rod can beelongated from about 50 mm to about 500 mm, with typical core diametersof 1-5 mm and overclad diameters of between 2 mm and 10 mm. In a typicalexample, a tapered core rod has minimum diameter of about 2.2 mm, atotal length of about 5.8 mm, and can be used to produce a fiber about 5m long. Tapered cores in a core rod are typically periodically taperedwith a period of between 1 mm and 20 mm and taper periods in drawnfibers can range from about 10 mm to 50 m. Sleeving tubes of variousdimensions (diameters, wall thickness) can be used. For example, asleeving tube having a 6.5 mm inside diameter and a 19.6 mm outsidediameter can be used. In typical examples, tapered core rods are sleevedwith silica tubes of inner diameters between 5 mm and 15 mm, andexterior diameters of 10 mm to 25 mm.

In a particular example, a 25 mm long core rod with a core having a 5 mmdiameter and an overclad having a diameter of 15 mm is situated in asleeve and elongated to be about 200 mm long and the core and overcladhave diameters of 1.75 mm and 5.3 mm, respectively. The core rod istapered to form tapers having a minimum diameter of 2.2 mm and a lengthof 5.8 mm. In one example, such a tapered core rod is sleeved with atube having a 6.5 mm inside diameter and a 19.6 mm outside diameter, anddrawn to a 5 m long fiber. In other examples, the sleeve is collapsed onthe tapered core rod, and ground or etched to form a substantiallyuntapered rod of length of about 200 mm, and an outside diameter ofabout 13 mm. The untapered rod can be re-sleeved with additionalsleeving tubes to increase preform and fiber diameters.

Example 12

The examples disclosed above are representative, and are selected forconvenient illustration, and many other examples can be provided. Forexample, preforms and fibers having oval, elliptical, polygonal (such ashexagonal and octagonal) cross sections can be provided. While dopedcores are convenient for forming optical waveguides, claddings can bedoped as well to provide suitable refractive index profiles, or bothcores and claddings doped. In most guided wave examples, the core andcladding are doped or otherwise configured so that a refractive index inthe core is larger than a cladding refractive index. In some examples,the core and/or the cladding are actively doped with one or more activematerials so as to form lasers, or optical amplifiers. For example,rare-earth elements such as erbium, ytterbium, neodymium, dysprosium,praseodymium, and thulium can be included. However, other configurationsare possible. Sensitizing agents to promote pumping can also be added,and stress rods or other features associated with polarizationmaintaining waveguides can be provided.

Example 13

With reference to FIG. 10, a representative method of fiber manufactureincludes directing a core rod, core tube, or other core precursor 1002situated within one or more sleeving tubes such as the sleeving tube1004 to a draw furnace 1006. The core precursor 1002 and the sleevingtube 1004 can be fed to the draw furnace 1006 at feed rates Vcore,Vclad, respectively. These feed rates can be varied during drawing and adifference between these feed rates can be constant or time varying.After fusing in the draw furnace 1006, the fused core precursor/sleevingtube form a fiber 1008 that exits the draw furnace at a draw speedV_(fiber). By selecting different feed rates for the core precursor 1002and the sleeving tube 1004, the fiber 1008 can be provided with aconstant or variable taper. In some examples, the feed rates can bedetermined based on pressures applied to one or more of the coreprecursor 1002 and the sleeving tube 1004, or volumes within the coreprecursor 1002 or the sleeving tube 1004.

Having described and illustrated the principles of the disclosedtechnology with reference to the illustrated embodiments, it will berecognized that the illustrated embodiments can be modified inarrangement and detail without departing from such principles. We claimall that is encompassed by the appended claims.

We claim:
 1. An optical fiber, comprising: a tapered core that extendsalong a propagation axis, wherein the tapered core is at least partiallydoped with a rare earth element; and an inner cladding that surroundsthe core and extends along the propagation axis, wherein the taperedcore and the inner cladding have respective cross-sectional areasA_(CORE), A_(INNER), and a ratio A_(CORE)/A_(INNER) varies along thepropagation axis, wherein the tapered core and the inner cladding definea plurality of portions with few mode cores and a plurality of portionsthat define single mode waveguides, wherein the single mode waveguideportions and the few mode core portions are spatially separate andalternatingly disposed along the propagation axis, and wherein an outeredge of the inner cladding defines a constant cross-sectional area. 2.The optical fiber of claim 1, further comprising an outer cladding thatsurrounds the inner cladding, wherein the inner cladding and the taperedcore are configured to guide optical radiation in the inner cladding. 3.The optical fiber of claim 1, wherein the tapered core and the innercladding have circular cross-sectional areas.
 4. The optical fiber ofclaim 1, wherein a radius of the tapered core varies linearly along thepropagation axis.
 5. The optical fiber of claim 1, wherein a core radiusof the tapered core varies periodically along the propagation axis. 6.The optical fiber of claim 1, wherein a core radius of the tapered corevaries quasi-periodically along the propagation axis.
 7. The opticalfiber of claim 1, wherein the tapered core is centered with respect tothe inner cladding.
 8. A method, comprising: situating a core rod havinga tapered core in a cladding tube, wherein the tapered core is dopedwith an active dopant; and drawing the core rod and the cladding tube soas to form a few mode optical fiber having an inner cladding thatsurrounds a fiber core and extends along the propagation axis, whereinthe tapered core and the inner cladding have respective cross-sectionalareas A_(CORE), A_(INNER), and a ratio A_(CORE)/A_(INNER) varies alongthe propagation axis, wherein the fiber core has a plurality of portionswith few mode cores and a plurality of single mode tapered portions,wherein the single mode tapered portions and the few mode core portionsare spatially separate and alternatingly disposed along the propagationaxis, and wherein an outer edge of the inner cladding defines a constantcross-sectional area.
 9. The method of claim 8, wherein the core rod andthe cladding tube are drawn so that the optical fiber has a constantcladding diameter.
 10. The method of claim 8, wherein the core rod has aconstant outside diameter.
 11. An optical fiber, comprising: a taperedcore that extends along a propagation axis, wherein the tapered core isat least partially doped with a rare earth element; and an innercladding that surrounds the core and extends along the propagation axis,wherein the tapered core and the outer edge of the inner cladding definerespective cross-sectional areas A_(CORE) and A_(INNER) having diametersd1 and d2, respectively, wherein d1 varies along the propagation axisand each of d1 and d2 varies periodically along the propagation axisover at least two periods, and wherein the tapered core and the innercladding define a few mode core, and a portion of the tapered coredefines a single mode waveguide.
 12. The optical fiber of claim 11,wherein the periodic variation is a sinusoidal variation at a fixedspatial frequency.
 13. The optical fiber of claim 11, wherein theperiodic variation is associated with a chirped spatial frequency.